1. Field of the Invention
The present invention relates to an apparatus for communicating with an RFID (Radio Frequency Identification) tag through a radio wave (hereinafter, referred to as tag communication apparatus), an antenna used for the tag communication apparatus, a tag communication system, a scanning adjusting method for tag communication apparatus and a computer readable medium for scanning adjustment.
2. Description of the Related Art
In recent years, technology for RFID (Radio Frequency Identification) in which a tag communication apparatus as a reader and/or writer performs a radio communication with an RFID (Radio Frequency Identification) tag (radio tag) has been progressively employed. Further, the RFID tag is anticipated especially in a field of physical distribution as an alternative of a bar code. Thus, the radio tag is expected to be widely used in near future.
As a communication system between the RFID tag and the tag communication apparatus, there are an electromagnetic induction system and a microwave system. The electromagnetic induction system is used in a frequency band such as a band from 125 k to 135 kHz or a 13.56 MHz band. On the other hand, the microwave system is used in a frequency band such as a 2.45 GHz band, and is considered to be used in what is called an UHF band of approximately from 800 MHz to 960 MHz.
Currently, the electromagnetic induction system has been progressively spread. However, generally, it is easier to extend a communication distance in the microwave system than in the electromagnetic induction system. Further, as the frequency band becomes higher, the size of an antenna of the RFID tag can be more reduced in the microwave system than in the electromagnetic induction system. Therefore, the microwave system is anticipated to be widely used in near future. Accordingly, an RFID tag and a tag communication apparatus of the microwave system have been also progressively developed.
As described above, in the microwave system, the communication distance between the tag communication apparatus and the RFID tag can be easily extended to from about several centimeters to about several meters compared to the electromagnetic induction system. Accordingly, a communication area which is an area where the tag communication apparatus can communicate with the RFID tag can be easily enlarged. However, when the communication area is enlarged, following problems arise.
As an antenna for tag communication which is an antenna of the tag communication apparatus, cases may be considered in which a nondirectional antenna or an antenna having low directivity is used and in which an antenna having high directivity is used. For instance, in JP-A-2002-151944, an RFID system using a Yagi antenna having high directivity is described. However, the communication area of the antenna having high directivity is narrower than that of the antenna having low directivity. Thus, it is difficult to construct a system in which a number of RFID tags can be read and written simultaneously with a small number of antennas.
On the other hand, as a configuration of a system in which a number of RFID tags are read and written simultaneously, configurations of antennas as shown in FIGS. 13 and 14 are generally considered. FIG. 13 shows a communication area when the antenna having low directivity is used. FIG. 14 shows a communication area when the antenna having high directivity is used.
When the antenna having low directivity is employed, a wide communication area 101 can be covered with a small number of antennas 100. However, when the wide communication area 101 is covered with the small number of antennas 100, a number of RFID tags 102 exist in the communication area 101 at the same time. Accordingly, a collision of communication arises between the small number of antennas 100 and the number of RFID tags 102 for a number of times, and the quality of the communication is deteriorated.
Further, when the antenna having low directivity is employed, a incommunicable region in which the tag communication apparatus cannot communicate with the RFID tag is generated in the communication area due to what is called a multipath interference, in which direct waves from the tag communication apparatus and reflected waves from a floor surface or a wall surface interfere with each other. In this case, the RFID tag that cannot communicate with the tag communication apparatus may possibly be generated in the communication area, which is undesirable. Further, there is a high possibility that a communicable region in which the tag communication apparatus can communicate with the RFID tag is generated outside the communication area due to the multipath interference. This is not preferable when the communication area is desired to be limited to a desired area.
To overcome the above-described problem, it is considered that a position, a direction and an output of the antenna are adjusted so that the incommunicable region is not generated. However, since this adjustment needs to be carried out while confirming that the communication can be performed in each part inside the communication area, a lot of time and labor are required.
On the other hand, when an antenna 110 having high directivity is used as shown in FIG. 14, a communication area 111 covered with one antenna is narrower than that when the antenna 100 having low directivity is used as shown in FIG. 13. Thus, the generation of the collision of communication can be suppressed, and the deterioration of the quality of the communication can be suppressed. Further, when the antenna 110 having high directivity is used, an intensity of a radio wave propagated to the floor surface or the wall surface is suppressed. Thus, the problem generated by the multipath interference is avoided.
However, in order to cover a wide communication area with the antenna 110 having high directivity, a number of antennas are necessary as shown in FIG. 14. Further, the size of the antenna 110 having high directivity is larger than that of the antenna 100 having low directivity. Accordingly, the size of the antenna of the tag communication apparatus is extremely enlarged.